Tub Sub is a windup bathtub toy. Its chassis is based on an US Los Angeles-class nuclear submarine (minus the conning tower). The winding handle is built into the port plane. It is wound to provide energy for the single screw, and must be pushed in to engage the gears and drive mechanism. In addition, there is a small hole in the chassis that is stopped by a plastic button (much like those in small plastic water guns). The child can remove the stopper to allow water into the sub and can experiment with different water levels and their effect on the submarine’s buoyancy.
Children would be most entertained by this toy during bath time. The screw propels the sub through the water and agitates and splashes the water, much to a young child’s content. It should be handled by children from five to ten years of age because of the possibility of the screw breaking off and being swallowed. Experimenting with buoyancy could keep even a pre-teen’s attention in a tub or swimming pool, especially if there are multiple children and subs in the water.

Constructing Tub Sub would be a fairly trivial matter since it has little need for precision or expensive parts. Plastic and stainless steel would do just fine and last extending periods of submersion and play.
The only real safety concern comes from the dismantling of Tub Sub. The small gears, drive shafts, and screw could choke a younger child if put in their mouth. With normal use however, the toy should withstand years of play without breaking or injuring the child. By the time Tub Sub fell apart, most children would probably have stopped playing with it or learned not to put toys in their mouths.